This invention relates to automatic shoe clearance adjusting devices in shoe drum brakes for use in vehicles such as automobiles or the like.
Automatic shoe clearance adjusting devices are generally mounted in shoe drum brakes for adjusting the clearance between the shoe and the drum to a desired value automatically when the clearance has increased according to wear of shoe lining.
One prior art automatic shoe clearance adjusting device comprises a strut of adjustable length including a first strut member having a hollow portion for slidably receiving a male thread portion of a second strut member, an adjust nut having ratchet teeth on the outer periphery thereof and engaging with the male thread portion of the second strut member with one side surface of the adjust nut abutting with an open end of the hollow portion of the first strut member. An adjust lever is pivotally connected to one end of the strut (one end of either of the first and second strut members) and is pivotable in response to the actuation of a hand brake lever so as to engage with the adjust nut to rotate it when shoe clearance exceeds a predetermined value, thereby maintaining a suitable shoe clearance by rotating the adjust nut relative to the second strut member.
In the aforementioned prior art shoe clearance adjusting device, it is necessary to arrange the adjust lever such that the adjust lever can escape from the adjust nut when the adjust lever returns to its original position after rotating the adjust nut (in a direction increasing the effective length of the strut) to prevent the reverse rotation of the adjust nut, and that the adjust lever can escape from the movement of the hand brake lever (having some amount of free movement relative to the hand brake lever) so as to permit the actuation of the hand brake lever even when the adjust lever cannot rotate because of rust or the like.
It has been proposed to mount the adjust lever pivotally on the hand brake lever with a coil spring acting therebetween so that the adjust lever is urged against the adjust nut by the compressive force of the coil spring and is urged in the direction of rotational movement of the hand brake lever by the torsional force of the coil spring. The adjust lever can escape in respective opposite directions when the coil spring deflects in respective opposite directions.
In the heretofore proposed devices, the torsional force of the coil spring acts to cause excessive frictional force between the adjust lever and the hand brake lever when the adjust lever escapes from the adjust nut against the compressive force of the coil spring, which tends to cause insufficient escape movement of the adjust lever from the adjust nut, thereby resulting reverse rotation of the adjust nut.
Further, in the heretofore proposed devices, the coil spring does not necessarily contact at a predetermined position relative to the adjust lever for transmitting compressive force. Thus, it has been difficult to obtain a stable urging force acting on the adjust nut. Further, parallelism between the hand brake lever and the adjust lever will sometimes be impaired by the compressive force of the coil spring and clearance adjustment will not be performed.
An object of the present invention is to remove the shortcomings of the prior art automatic shoe clearance adjusting device of the aforementioned type by a special construction of the adjust lever and coil spring arrangement.